In the construction of semiconductor integrated circuits electrical resistors are often provided as thin films on a surface of a semiconductor body. Due to the limited resistivity of such films and the very small space available on a typical integrated circuit chip, only resistors with comparatively small resistance values may be so provided. Larger resistances are typically provided as discrete elements external to the integrated circuit. Such external resistors take up a greater amount of space than integrated film resistors, are typically more expensive to provide, and are inherently less reliable because of the requirement for mechanical interconnections.
A second problem relates to the stability of the electrical resistance of a film. Typically such a film is annealed after deposition. If at a later time the film is heated to a temperature approaching the annealing temperature the resistance value can shift significantly. Film resistors may reach such temperatures when operating in a hot environment. The heat from the ambient conditions combined with heat generated within the semiconductor body and Joule heating of the resistor can drive the actual temperature of the film to high values. Such shifts in resistance values may degrade the performance of a circuit including the resistor or may even prevent the circuit's operation altogether.